Niobium pentoxide-containing borosilicate glasses

ABSTRACT

Niobium pentoxide-containing borosilicate glasses having a medium to high index of refraction, a relatively high dispersion, and a high chemical resistance, consist of 30 % TO 42 %, BY WEIGHT, OF THE GLASS-FORMING COMPONENTS SILICON DIOXIDE AND BORON TRIOXIDE, THE SILICON DIOXIDE CONTENT THEREOF BEING BETWEEN 22 % AND 30 %, BY WEIGHT, AND THE BORON TRIOXIDE CONTENT THEREOF BEING BETWEEN 8 % AND 14 %, BY WEIGHT, 2 % TO 53 %, BY WEIGHT, OF NIOBIUM PENTOXIDE, AND 9 % TO 22 %, BY WEIGHT, OF POTASSIUM OXIDE. These glasses may additionally contain other metal oxides of fluorides in predetermined amounts.

United States Patent Broemer et al.

[ 1 Apr. 15, 1975 NIOBIUM PENTOXIDE-CONTAINING BOROSILICATE GLASSES [75] Inventors: Heinz Broemer, Hermannstein;

Norbert Meinert, Wetzlar, both of Germany [73] Assignee: Ernst Leitz GmbH. Optische Werke, Wetzlar, Germany [22] Filed: Nov. 27, 1972 [21] Appl. No.: 309,804

[52] U.S. Cl. 106/54; 106/53; 106/47 Q [51] Int. Cl. C03c 3/08 [58] Field of Search 106/54, 47 Q [56] References Cited UNITED STATES PATENTS 2,576,521 11/1951 Kreidl 106/54 3,460,954 8/1969 Young 3,503,764 3/1970 Young 3,716,385 2/1973 Ritze 106/54 X Primary ExaminerWinston A. Douglas Assistant ExaminerJohn F. Niebling Attorney, Agent, or FirmErich M. H. Radde [5 7 ABSTRACT Niobium pentoxide-containing borosilicate glasses having a medium to high index of refraction, a relatively high dispersion, and a high chemical resistance, consist of 30 to 42 by weight, of the glass-forming components silicon dioxide and boron trioxide, the silicon dioxide content thereof being between 22 and 30 by weight, and the boron trioxide content thereof being between 8 and 14 by weight, 2 to 53 by weight, of niobium pentoxide, and

9 to 22 by weight, of potassium oxide. These glasses may additionally contain other metal oxides of fluorides in predetermined amounts.

22 Claims, No Drawings NIOBIUM PENTOXIDE-CONTAINING BOROSILICATE GLASSES BACKGROUND OF THE INVENTION 1. Field Of The Invention The present invention relates to borosilicate glasses and more particularly to borosilicate glasses containing niobium pentoxide, said glasses having improved prop- In principle these objects of the present invention are achieved by providing a glass which consists to about 30 7: to 42 71. by weight, of the glass-forming components silicon dioxide SiO and boron trioxide B In such glass compositions the silicon dioxide content must be between about 22 7t and about 30 by weight, and the boron trioxide content between about 8 7c and about 14 72, by weight. In addition thereto these glass compositions contain between about 2 7( emei and to a Process of Producing Sameand about 53 7c, by weight, of niobium pentoxide and 2. Description Of he Prio Art between about 9 7: and about 22 7t. by weight, of po- Glass for optical purposes which is obtained by meltiassium id ing batches of mixtures consisting of silicon dioxide. According to an advantageous embodiment of the boron trioxide, and alkali metal oxides are not only present invention the mixtures of components from known but have also been thoroughly investigated in l5 which the glasses are produced by melting, can additechnical and scientific respect for a long time. Such tionally contain up to 38 7:, by weight, of an oxide seglass batch mixtures. as is known. can easily be melted lected from an element consisting of lithium, sodium, down and yield optical glasses which are substantially magnesium calcium, strontium, barium, zinc, cadfree of striae or schlieren and bubbles without particu- 7O mium, lead, min in i m. l n h m, talum, l h i l expenditure, antimony, bismuth, tellurium, and titanium; or. respec- The disadvantage of' these known optical glasses retively, p to y welghtsides essentially in the fact that they are relatively friomthefluol'lflesofelefnems0l1$1tmgofllthlumrso' readily attacked by the atmosphere. Therefore, atdlum magnesium Calclumv Stmntlum, barlumt lead, tempts have been made to further modify the composi- 75 and alumlnumtion of such glasses by adding thereto oxides of ele- Acfiordmg to another embPdlmem of the p ments of the second and third groups of the periodic f there can be added; m Place of the q an system. and particularly by adding thereto Calcium alkali metal-hydrogen fluoride or a complex alkali metoxide and a larger or smaller amount of aluminum oxal'hexafluoro 5 mg The hydrolytic type of the glasses was improved Glass compos1t1ons according to the present invensomewhut by the addition of these Oxides tion have the further advantage that the startmg batch Furthermore, glasses of similar optical properties are imxttures be zfi i i g g fi l known. namely glasses of the heavy flint type which 3 h? e g 3 bbl e P; th fth consist essentially of silicon dioxide and lead monoxide m y 0 u 1 g' or of silicon dioxide. lead monoxide. and lanthanum 5 a proper 0 e resu g asses ee 9 s a ttally all the present day requirements of the optical deoxide. However. such glasses and espec1ally those with u signer, so that a wide range of use in the manufacture a higher content of lead monoxide have the disad anof optlcal instruments is afforded. tage that they are, 1n general. rather sensitive to staming. Therefore. considerable difficulties in the large- DESCRlPTlON OF THE PREFERRED scale manufacture of lenses are encountered. 40 EMBODIMENTS SUMMARY OF THE INVENTION The following examples serve illustrate the present invention without, however, limiting the same thereto. one object the Present P P Q to P The attached Tables illustrate the composition of use- QP glflssesbavmsmedlym Q f- 0f refruc' ful glasses according to the present invention. In these and relatively hlgh dlspel'slon Whlch glasses are Tables the amounts of the various components are substantially free of the disadvantages of the known opgiven in per cent by weight. tical glasses and which haveahigh chemical resistance. I T bl I th are t forth gl ss compositions are substantially colorless. and can readily be further which nsist exclusively of the four main components processed into blanks and molded bodies. silicon dioxide SiO boron trioxide B 0 potassium Another ob ect of the present 1nvent1on is to provlde oxide 0, and niobium pentoxide Nb O The actual a simple and effective process of producing such valuglass-forming components (SiO. ,-l- B 0 are present in able optical glasses. an amount between about 30 71 and about 42 7(. by

Other objects and advantageous features of the presweight. The proportion of these glass-forming compoent invention will become apparent as the description nents is in the lower part of the indicated range when roceeds. a hi h index of refraction is desired. p 2

TABLE 1 Example I 2 3 5 6 $10 30.0 28.5 28.5 25.9 23.5 22.4 B 0, 11.0 13.5 10.5 9.5 8.6 8.2 K 0 22.0 21.0 21.0 19.0 17.2 10.4 Nh. .0 37.0 37.0 40.0 45.6 50.7 53.0 6,. 1.6748 1.6868 1.7068 1.7590 1.7775 1.7942 9,. 30.3 51.5 29.6 26.3 25.5 24.4

Table l. furthermore, shows that a decrease of the potassium oxide content yields a glass of a higher index of refraction and that an increase of the niobium pentoxide content yields also a glass of a higher index of refraction. Surprisingly it was found that the glasses produced by melting such mixture are colorless as well as metal oxides and the variations in the indices of refraction Ne and the Abbe values (11,.) obtained thereby.

In this case also, with the exception of the melts of Example 23, the amounts of the glass-forming components and of the alkali metal oxides were not varied so as to clearly show the possibilities of modifying the oplow in schlieren and bubbles even when they have a high content of niobium pentoxide Nb- O Table 11 shows that in addition to potassium oxide K 0, lithium oxide Li- O or sodium oxide Na- O can also tical properties as a function of the addition of said trivalent and tetravalent metal oxides.

Examples 20 to 24 show that with an increasing content of titanium dioxide TiO the index of refraction be used' and the dispersion of the glasses increase greatly. These TABLE 11 glasses, despite their high titanium dioxide content, are Example: 3 7 8 9 surprisingly little colored and relatively stable. As a resalt thereof they are of considerable interest to the op- $10 28.5 28.5 28.5 24.7 tical designer. B 0 10.5 10.5 10.5 9.0 K 2 0 m 3m 9,0 In place of titanium dloxrde, titanium can also be 1nfi 0 cluded in the mixture in the form of other compounds, 'j I 7 J particularly in the mixture in complex form as alkali n, 1.706 1, 030 7 r7 20 metal hexafluorotitanate, for instance, as potassium 409 hexafluorotitanate K TiF Table V shows the effect of the replacement of niobium pentoxide by potassium hexafluorotitanate. In the examples given in Table V the amounts of the three main components (silicon dioxide SiO- boron trioxide B 0 and potassium oxide K 0) were not varied. Only Partial replacement of niobium pentoxide Nb O by tantalum pentoxide Ta- O produces a glass which is very similar in its chemical properties and in those properties pertaining to glass technology. This is shown in Table 111. Thereby, the amounts of the two glasforming components and the alkali metal oxide were not varied. It is evident from Table 111 that the index of refraction decreases with an increase in the tantalum pentoxide content Ta. ,O which replaces the proportionally decreasing niobium pentoxide Nb- O content.

the amounts of niobium pentoxide Nb O and of potassium tetrafluorotitanate K TiF were systematically varied. whereby the sum of Nb O and KJiF always equals 40 7:. by weight.

In accordance with a further embodiment of the present invention, fluorides of elements selected from the TABLE 111 group consisting of lithium, sodium, magnesium, calcium. strontium, barium, lead, and aluminum as well as Example H l2 potassium bifluoride KHF can partially replace nio- Siou 385 23.5 385 3&5 bium pentoxide Nb O while the amounts of the three 22g. other glass-forming components of the glass composi- Nihoi M 5 b tions, namely silicon dioxide, boron trioxide, and potas- Ta. .o 10.0 20.0 30.0 311.0 sium oxide are not varied. Examples of such glass coml-6874 M733 l-6535 L642) positions are given in Table VI. It is evident that even 1 32.4 34.3 37.4 39.7

a relatively small amount of niobium pentoxide in the glass composition is sufficient to produce glasses of a Table [V shows the use of trivalent and tetravalent good hydrolytic type.

TABLE IV Example 14 15 16 17 l8 19 20 21 22 23 2-1 SiO 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 13 0,. 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 13.5 10.5 1\. .O 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 Nh. 30.0 20.0 10.0 30.0 30.0 20.0 30.0 20.0 10.0 2.0 5.0 Sh 10.0 20.0 30.0 10.0 T00; 10.0 20.0 T10 10.0 20.0 30.0 35.0 35.0 'n,. 1.6867 1.6938 1.6331 1.6829 1.6359 1.6590 1.7131 1.7302 1.7443 1.7472 1 7656 1 32.3 31.2 33.7 32.7 37.7 35.3 28.5 26.5 24.6 24.4 23.5

TABLE V1 Example 31 32 33 34 35 36 37 3s 39 40 S102 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 B20 10.5 10. 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 K 0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 N620, 30.0 30.0 30.0 20.0 30 0 30.0 20.0 30.0 20.0 10.0 LiF 10.0

NaF 10.0

KHF Mgln 10.0 20.0 CaF- 10.0 SrF. 0 0 1361-1. 10.0 1.0 30.0 PhF. AIR, n,. 1.6657 1.6386 1.6382 1.6127 1.6551 1.6626 1.6268 1.6642 1.6380 1.6066 11,. 35.7 37.3 36.2 41.1 36.8 36.3 41.9 36.4 41.2 48.2

E. ample -11 42 43 44 45 46 47 510 214.5 211.5 221.5 28.5 28.5 221.5 221.5 13. 0 10.5 10.5 10.5 10.5 10.5 10.5 10.5 K 0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 mp, 0 30.0 20.0 10.0 30.0 20.0 10.0 LiF NaF KHF. 10.0 20.0 30 0 MgF (31F.

SrF- BaF PM 10.0 20.0 0.0

AIF 10.0 11,. 1.6234 1.6442 1.5874 1.5410 1.6760 1.6617 1.6298 v. 34.5 36.1 43.3 49.9 32.9 34.5 37.1

A valuable aid in establishing desired optical values TABLE VIII -without impairing the stability and resistance of the glassis afforded by the partial replacement of niobium Example 61 62 63 64 65 pentoxideNb o by an ox1de of the elements magne- 5 so 2875 285 28.5 2845 285 s1um. calcium. stront1um, banum, zinc, cadmlum, and )5 m5 m5 105 105 lead. These oxides are added in an amount up to about K20 21.0 21.0 21.0 31.0 21.0 30 7!. by weight. On exceeding said limit. especially 'fi" Q" 2" when using lead monoxide PbO to replace niobium P6 0}, 10.0 300 pentoxide Nb O the glasses produced by melting such t- .1 1 (6; 1 (gm I 6: 238 :11 37 n. .6 compositions, become softer. Table V11 glves a number w 34.3 34.2 4U 353 445 of examples of compositions of th1s type.

TABLE V11 $102 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 28.5 13.0.. 10.5 10 5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 1 0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 21.0 Nb. .O-, 30.0 30.0 10.0 30.0 10.0 30.0 10.0 30.0 10.0 30.0 10.0 30.0 10.0

MgO 10.0 C60 10 0 30.0 SrO 10.0 30.0 BaO 10.0 30.0 Zno 10.0 30 0 o 10.0 30.0 PbO 10.0 30.0

Trivalent metal oxides can also be used successfully in glass batch compositions in accordance with the present invention. Table V111 gives examples of such glass compositions in which part of the niobium pentoxide has been replaced by aluminum oxide A1 0 in dium oxide ln O or lanthanum oxide La O From the foregoing examples it is observed that the index of refraction (n,.) for the glass compositions of the present invention ranges between a value of 1.5410

and 1.7942, whereas the Abbe value (n,.) for the composition ranges between 23.5 (v,,,,,, 49.9. It is observed that all of the compositions in accordance with the invention contain between 30 and 42 weight percent of the glass forming components SiO- and B 0, and between 9 and 22 weight percent of K however, when the compositions contain Nb O as the only additional component, the Nb O is present within the range of 37 to 53 weight percent as illustrated in Table I. On the other hand, when part of the Nb O is replaced with a metal oxide or a fluorine containing compound, the compositions contain between 37 and 58 weight percent of the combined Nb O and additional compound, which in turn is observed to constitute between 2 and about 48 weight percent of Nb O with the balance being made up by one of the additional components. The permissible amounts of additional components may be summarized as follows:

Up to weight percent of an oxide of the elements Li, Mg, Al, and Bi;

up to weight percent of an oxide of the elements Na and Te;

up to weight percent of an oxide of the elements Ca, Sr. Ba, Zn, Cd, Pb, In, La, and Sb; up to weight percent TiO up to 38 weight percent Ta O up to 10 weight percent of a fluoride of the elements Li, Na, Ca. and Al;

up to 20 weight percent of a fluoride of the elements Mg and Sr;

up to 30 weight percent of a fluoride of the elements Ba and Pb;

up to 30 weight percent KHF and up to 30 weight percent K TiF The glass according to the present invention is pref-v erably produced by melting compositions as given in the attached Tables in platinum vessels at a temperature between about 1,200 C. and about l,300 C. After melting down the starting batch, fining and homogenizing is effected at 1400 C., whereupon the melt is cooled to pouring temperature. Pouring and casting are then carried out in the conventional manner.

As stated hereinabove, the glasses according to the present invention have improved chemical and physical properties. Due to these improved properties the range of application of said glasses is very considerably increased. Thus they.can be used as external or front lenses in optical systems of all kinds, for instance, of binoculars. objectives of cameras and microscopes, prism systems, and in general for all optical systems of high value in which an optical glass of the advantageous and specific properties of the glasses according to the present invention is required.

Especially noteworthy is the low susceptibility of the glasses according to the present invention against staining or spotting as determined by allowing a standard acetate solution of the pH-value of 4.6 and/or a sodium acetate buffer solution of the pH-value of 5.6 to act upon the plane-polished surface of the glass to be tested. Thereby, interference color spots are produced more or less rapidly due to decomposition of the glass surface.

The hydrolytic properties of the glasses of the present invention such as resistance against the changes in climate, i.e. changes in the atmospheric humidity and temperature, and their weathering resistance are also considerably improved.

We claim:

1. A niobium pentoxide-containing borosilicate glass composition having an index of refraction (n,.) between 1.5410 and 1.7942, an Abbe value (11,.) of between 23.5 and 49.9 and being characterized by a high degree of chemical resistance, consisting essentially of A. from about 30% to about 42% by weight of the combined glass-forming oxides SiOz and B 0 wherein the SiO2 content of said combined amount comprises between about 22% and about 30% by weight and the B 0 content of said combined amount comprises between about 8% and about 14% by weight;

B. between about 9% and about 22% by weight of C. and a component selected from the group consist- .ing of i.) between about 37% and about 53% by weight of bib- 0 and ii.) from about 37% to about 58% by weight of a combination of Nb O and a further component, wherein the Nb O content in said combined amount comprises between about 2% and about 48% by weight and said additional component in combination with the Nb O is selected from the group consisting of a. an amount of an oxide of the element Li, Mg, Al

or Bi, said amount not substantially exceeding about 10% by weight.

b. an amount of an oxide of the element Na or Te, said amount not substantially exceeding about 20% by weight;

c. an amount of an oxide of the element Ca, Sr, Ba,

Zn, Cd, Pb, 1n, La or Sb, said amount not substantially exceeding about 30% by weight;

d. an amount of TiO- said amount not substantially exceeding about 35% by weight;

e. an amount of Ta O said amount not substantially exceeding about 38% by weight;

f. an amount of a fluoride of the element Li, Na, Ca or Al, said amount not substantially exceeding about 10% by weight;

g. an amount ofa fluoride of the element Mg or Sr, 'said amount not substantially exceeding about 20% by weight;

h. an amount ofa fluoride of the element Ba orPb, said amount not substantially exceeding about 30% by weight;

i. an amount of KHF said amount not substantially exceeding about 30% by weight; and

j. an amount of K TiF said amount not substantially exceeding about 30% by weight.

2. The glass composition as defined by claim 1 wherein C. is the component consisting essentially of from about 37% to about 53% by weight of Nb O 3. The glass composition as defined by claim 1, wherein C. is the component consisting essentially of from about 37% to about 58% by weight of the combination of Nb O and an additional component.

4. The glass composition as defined by claim 3, wherein said additional component is lithium oxide.

5. The glass composition as defined by claim 3, wherein said additional component is sodium oxide.

6. The glass composition as defined by claim 3, wherein said additional component is magnesium oxide.

7. The glass composition as defined by claim 3, wherein said additional component is zinc oxide.

8. The glass composition as defined by claim 3, wherein said additional component is cadmium oxide.

9. The glass composition as defined by claim 3, wherein said additional component is lead monoxide.

10. The glass composition as defined by claim 3,

wherein said additional component is bismuth trioxide.

11. The glass composition as defined by claim 3, wherein said additional component is aluminum oxide.

12. The glass composition as defined by claim 3, wherein said additional component is titanium dioxide.

13. The glass composition as defined by claim 3, wherein said additional component is tellurium dioxide.

14. The glass composition as defined by claim 3, wherein said additional component is tantalum pentoxide.

15. The glass composition as defined by claim 3, wherein said additional component is lithium fluoride.

16. The glass composition as defined by claim 3, wherein said additional component is sodium fluoride.

17. The glass composition as defined by claim 3,

fluoro titanate.

UNITED STATES PATENT AND TRADEMARK OFFICE trrrrrrrrmrr or mrrrcrr PA'rrrrr NO. 3 877 953 OATH) April 15 1975 lNVENTGR(S) I I-IEINZ BROEMER and NORBERT MEINERT it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

After [21] Applr, Nos 309,804" the following insertion should be made:

[30] Foreign Application Priority Data December 2 1971 Germany P 21 59 759,1

Last line of ABSTRACT: "of" should read or Column 4,, line 2: "'Ne should read h "'a Table IV column of "Example"; "BiO should read Bi O Table VI column of "Example": "BeF should read BaF Table VIII column of "Example": 'Pn 0 should read In O -7 and "B C should read B 0 Column 6 lines 65-66 should read: "whereas the Abbe value (w for the composition ranges between 23. 5

and 4999 r'nrd an nineree' [SEAN y g August1975 Arrest:

RUTH C. MASON C. MARSHALL DAWN IPYIIIIX If/ F" (mnmzsxrmwr r Palenls and Trademurkx 

1. A NIOBIUM PENTOXIDE-CONTAINING BOROSILICATE GLASS COMPOSITION HAVING AN INDEX OF REFRACTION (NE) BETWEEN 1.5410 AND 1.7942, AN ABBE VALUE (VE) OF BETWEEN 23.5 AND 49.9 AND BEING CHARACTERIZED BY A HIGH DEGREE OF CHEMICAL RESISTANCE, CONSISTING ESSENTIALLY OF A. FROM ABOUT 30% TO ABOUT 42% BY WEIGHT OF LTHE COMBINED GLASS-FORMING OXIDES SIO2 AND B2O3, WHEREIN THE SIO2 CONTENT OF SAID COMBINED AMOUNT COMPRISES BETWEEN 22% AND ABOUT 30% BY WEIGHT AND THE B2O3 CONTENT OF SAID COMBINED AMOUNT COMPRISES BETWEEN ABOUT 8% AND ABOUT 14% BY WEIGHT; B. BETWEEN ABOUT 9% AND ABOUT 22% BY WEIGHT OF K2O, C. AND A COMPONENT SELECTED FROM THE GROUP CONSISTING OF I.) BETWEEN ABOUT 37% AND ABOUT 53% BY WEIGHT OF NB2O5 AND II.) FROM ABOUT 37% TO ABOUT 58% BY WEIGHT OF A COMBINATION OF NB2O5 AND A FURTHER COMPONENT, WHEREIN THE NB2O5 CONTENT IN SAID COMBINED AMOUNT COMPRISES BETWEEN ABOUT 2% AND ABOUT 48% BY WEIGHT AND SAID ADDITIONAL COMPONENT IN COMBINATION WITH THE NB2O5 IS SELECTED FROM THE GROUP CONSISTING OF A. AN AMOUNT OF AN OXIDE OF THE ELEMENT LI, MG, AL OR BI, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 10% BY WEIGHT. B. AN AMOUNT OF AN OXIDE OF THE ELEMENT NA OR TE, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 20% BY WEIGHT; C. AN AMOUNT OF AN OXIDE OF THE ELEMENT CA, SR, BA, ZN, CD, PB, IN, LA OR SB, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 30% BY WEIGHT; D. AN AMOUNT OF TIO2, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 35% BY WEIGHT; E. AN AMOUNT OF TA2O5, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 38% BY WEIGHT; F. AN AMOUNT OF A FLOURIDE OF THE ELEMENT LI, NA, CA OR AL, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 10% BY WEIGHT; G. AN AMOUNT OF A FLUORIDE OF THE ELEMENT MG OR SR, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 20% BY WEIGHT; H. AN AMOUNT OF A FLUORIDE OF LTHE ELEMENT BA OR PB, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 30% BY WEIGHT; I. AN AMOUNT OF KHF2, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 30% BY WEIGHT; AND J. AN AMOUNT OF K2TIF6, SAID AMOUNT NOT SUBSTANTIALLY EXCEEDING ABOUT 30% BY WEIGHT.
 2. The glass composition as defined by claim 1 wherein C. is the component consisting essentially of from about 37% to about 53% by weight of Nb2O5.
 3. The glass composition as defined by claim 1, wherein C. is the component consisting essentially of from about 37% to about 58% by weight of the combination of Nb2O5 and an additional component.
 4. The glass composition as defined by claim 3, wherein said additional component is lithium oxide.
 5. The glass composition as defined by claim 3, wherein said additional component is sodium oxide.
 6. The glass composition as defined by claim 3, wherein said additional component is magnesium oxide.
 7. The glass composition as defined by claim 3, wherein said additional component is zinc oxide.
 8. The glass composition as defined by claim 3, wherein said additional component is cadmium oxide.
 9. The glass composition as defined by claim 3, wherein said additional component is lead monoxide.
 10. The glass composition as defined by claim 3, wherein said additional component is bismuth trioxide.
 11. The glass composition as defined by claim 3, wherein said additional component is aluminum oxide.
 12. The glass composition as defined by claim 3, wherein said additional component is titanium dioxide.
 13. The glass composition as defined by claim 3, wherein said additional component is tellurium dioxide.
 14. The glass composition as defined by claim 3, wherein said additional component is tantalum pentoxide.
 15. The glass composition as defined by claim 3, wherein said additional component is lithium fluoride.
 16. The glass composition as defined by claim 3, wherein said additional component is sodium fluoride.
 17. The glass composition as defined by claim 3, wherein said additional component is magnesium fluoride.
 18. The glass composition as defined by claim 3, wherein said additional component is calcium fluoride.
 19. The glass composition as defined by claim 3, wherein said additional component is strontium fluoride.
 20. The glass composition as defined by claim 3, wherein said additional component is lead fluoride.
 21. The glass composition as defined by claim 3, wherein said additional component is aluminum fluoride.
 22. The glass composition as defined by claim 3, wherein said additional component is potassium hexafluoro titanate. 